A need exists for voltages higher than supply voltage levels in electronic devices. For instance, in order to modify non-volatile memory, such as flash memory, a high voltage level signal is needed for providing Fowler-Nordheim (FN) tunneling. High voltage levels may also be used for reading information stored in a memory cell or memory matrix in circuits otherwise operating at lower power supply voltage levels.
In electronic devices, high voltage signals are typically provided by charge pumps. Charge pumps are switched capacitor circuits which can provide a voltage level to a capacitive load up to (N+1)*Vdd, where N can be any number of stages in the charge pump and Vdd is the supply voltage. The supply voltage Vdd is typically 1.8 to 5.5 volts, but can be any other voltage level.
Charge pumps may be controlled by a plurality of clock signals or regulator circuits which control the desired charge pump output voltage level. Methods for regulating a charge pump include pulse-skip regulation and serial or linear regulation. In pulse-skip regulation, charge pump clock signals are enabled when the charge pump output voltage is lower than a desired value and disabled when the charge pump output voltage exceeds the desired value. In linear regulation, the charge pump output voltage is regulated by a closed-loop error amplifier and a pass device, such as a transistor. Linear regulation may provide a continuous adjustment of the charge pump output voltage, rather than the incremental and periodic adjustments provided by pulse-skip regulation.
FIG. 1A illustrates an example of a conventional high voltage level generator circuit 100 with pulse-skip regulation. Supply voltage Vdd is coupled to charge pump 102 which provides a high voltage level signal to load capacitance Cload 104. Operational amplifier (OP-AMP) 114 provides regulation by comparing the voltage divider level V1 at node 110 to voltage level VBGAP node 112, which may be a predetermined band-gap voltage level. The band-gap reference voltage VBGAP may be dependent on the materials used to fabricate an electronic device. The voltage level V1 is dependent upon the values of variable resistors R1 106 and R2 108. If V1>VBGAP, signal 116 is generated and an internal clock signal (not shown) in charge pump 102 is turned OFF, thereby disabling charge pump 102. If V1<VBGAP, a signal is generated on node 116 for enabling the internal clock signal to enable charge pump 102 to provide a high voltage level signal to Vout and capacitance Cload 104.
FIG. 1B illustrates an example of a conventional high voltage level generator circuit 101 with linear regulation. In circuit 101, a p-type metal-oxide semiconductor (PMOS) transistor 124 is coupled between node 122 and output node 126 which drives load capacitance Cload 128 with a high voltage level signal. A closed-loop amplifier configuration is provided by amplifier 134, transistor 124, and adjustable resistors R1 130 and R2 132. Voltage level VBGAP node 138 is the band-gap voltage level and V1 at node 136 is the voltage divider level. The high voltage level signal provided to Vout node 126 and capacitance Cout 128 is regulated using linear adjustment provided by the closed-loop configuration. The charge pump 120 provides the necessary supply voltage at node 122 for PMOS transistor 124.
In circuits 100 and 101, the high voltage level Vout provided by the charge pumps 102 and 120 is given by Equation(1) as follows:Vout=Req×VBGAP  Equation(1)Req, is given by Equation(2) as follows:
                              R          eq                =                                            R              1                        +                          R              2                                            R            2                                              Equation        ⁢                                  ⁢                  (          2          )                    
Therefore, the high voltage output Vout may be adjusted by changing the values of the variable resistors in circuits 100 and 101. The variable resistors may be configurable by using selection transistors which can enable or disable a resistor in series in a circuit, as desired, and provide real time selection of output voltages.
Problems may arise when providing the high voltage signal Vout to circuit elements across an electronic device. For example, in memory devices high voltage signals may be used on word-lines, bit-lines, source lines, a common transistor node, or any other node for programming or erasing information in a plurality of memory cells. The high voltage signals are supplied to these lines or nodes by controlled switches. Undesirable switching disturbances on supply lines in a memory device may be caused when a read operation takes place in at least one memory cell while a write or erase operation takes place in at least one other memory cell. Therefore, a need exists for improving electronic device operation by compensating for undesirable voltage supply line effects or disturbances.